Is there a canonical behavior for the behavior of an object that would have its weight/mass reduced below 0?

There is a level 10 spell in the Path of Air that “reduces the weight of an object by 60 pounds” (+20 lbs per added effect).

What happens if you cast this spell on an object that weighs, e.g., 59 lbs?

As a side note: the thing being reduced may actually be mass, given that the corresponding Weight Increment (Earth 10) spell uses as its unit the kilogram. Consider this interpretation as well.